The present invention generally relates to gas turbine engine exhaust noise suppression and, more particularly, to a quiet, low back pressure, exhaust eductor cooling system.
The air transportation industry has produced a widespread demand for low-noise gas turbine engine operations around aircraft. For example, in ground servicing of commercial aircraft, where ground crew fuel and provision the aircraft, load and unload baggage, and remove waste materials from the aircraft, certain noise level limits must not be exceeded in order to help ensure the health and safety of ground crew members. Under these ground service conditions, the propulsion engines of the aircraft are typically shut down, and only a turbine engine known as the auxiliary power unit (APU) remains in use. The APU can be used for in-flight operation as well. Examples of twin engine aircraft designed to use the APU include the Boeing 757, 767, and 777 and Airbus A300, A310, and A320. The APU gas turbine engine is usually located in a compartment in the aircraft tail structure. The APU can be used to generate electricity, furnish mechanical power from a rotating shaft, or provide pressurized air to the aircraft, for example, while the aircraft is being ground serviced. Pressurized air typically is used, for example, to power air cycle environmental control units (ECU), which heat or cool the aircraft passenger and crew cabins.
To maintain the noise level of the APU within acceptable limits, an apparatus is often provided to perform noise suppression. The apparatus may comprise an eductor system to entrain sufficient airflow through the compartment to perform necessary and desirable functions, such as APU cooling, compartment cooling, oil cooling, and providing an interface for the dumping of surge bleed control air. The apparatus may also comprise a noise suppression system to maintain the noise level of the APU within acceptable limits.
It is common for APU""s to provide compressed air flow which at times significantly exceeds the needs of the aircraft or the turbine. The excess flow, originating from within a load compressor driven by the turbine engine, is referred to as surge air and can be xe2x80x9cdumpedxe2x80x9d or vented into the ambient air. Venting of this pressurized air can significantly increase engine noise. In fact, this bleed noise can be expected to dominate the engine exhaust noise at high frequencies, i.e., above 2,000 Hz. In order to attenuate this air venting noise, many APU engine installations duct the surge dump air into the inlet of the APU turbine engine muffler in order that the muffler will attenuate the engine and surge air flow noise together sufficiently to meet noise emission standards, but results have been less than satisfactory.
It is desirable for the noise suppression apparatus to be able to perform any or all of the functions outlined above within a number of constraints. One constraint, for example, is that the apparatus operate with a minimum of back pressure to the APU turbine engine because back pressure to the turbine engine reduces the turbine engine efficiency, thereby requiring additional fuel burn for the APU to produce the same work. Another example constraint is that the weight of the noise suppression apparatus be minimized because weight is at a premium in aircraft, where extra weight reduces payload. Still another example constraint is that the noise suppression apparatus must be able to fit within the allocated volume of space inside the aircraft. Still another example constraint is that the apparatus should be appropriately oriented for efficient aircraft usage.
It is well known in the art that to achieve adequate noise suppression of an APU turbine engine, both its core noise and turbine noise must be attenuated. Core noise is a low frequency noise component caused primarily by the combustion process within the engine. Turbine noise is a high frequency noise component caused by the interaction of high velocity gases with the engine""s turbine section. The frequency spectrum of core noise is essentially broad band, peaking at relatively low frequency around 200 to 800 Hz where most of the sound energy of core noise is concentrated. Turbine noise, on the other hand, is a significantly higher frequency noise phenomenon, having both broad band and discrete spectral components, peaking at relatively high frequency around 15,000 to 20,000 Hz.
A combination of strategies can be used to simultaneously damp the core and turbine noise components. For example, the flow path of the hot gases from the core, turbine and eductor can be turned through a 90 degree angle to break up any direct acoustic path from the exit of the turbine engine to the exit of the tail pipe or noise suppressor exhaust. Also, acoustic treatment of the noise suppressor walls, including both bulk and reactive schemes, can be used to absorb acoustic energy, all as taught by U.S. Pat. Nos. 3,688,865 and 4,128,769. The noise suppression devices shown in these two patents are not suitable for use in aircraft, however, due to their large size. For example, the device of U.S. Pat. No. 3,688,865 is intended for ground based gas turbine engine test installations, and the device of U.S. Pat. No. 4,128,769 is intended for use in a ground vehicle or stationary ground installation. In addition, the turning of the exhaust flow through a 90 degree angle may present further problems in trying to fit these devices into the limited space available in aircraft, and in orienting the exhaust discharge along the direction of the centerline axis of the aircraft as is often desired.
Furthermore, as noise reduction requirements become more stringent in the future, and as the requirements for lighter, more efficient, smaller and less expensive engines increase, there will be a need for more efficient noise suppression which is adequate for use with different configurations of gas turbine engines, such as those with in-line combustors as well as those with reversed flow combustors.
As can be seen, there is a need for a quiet, low back pressure, exhaust noise suppression system. In particular, there is a need for a quiet, low back pressure, exhaust eductor system adequate to cool the APU compartment, the turbine engine casing and its associated gearbox, and generator oil. Additionally, there is a need for an exhaust eductor system that adequately attenuates the noise associated with flow through a surge bleed valve. Furthermore, there is a need for an exhaust eductor system that has minimal weight, is compact enough and configured for efficient aircraft use, and avoids large pressure losses to the gas turbine engine.
The present invention provides an APU compartment cooling system that also provides significant exhaust noise suppression. In particular, the present invention provides a quiet, low back pressure, exhaust eductor system which provides gas turbine engine and accessory cooling, compartment air cooling, oil cooling, and surge air noise suppression in addition to suppression of core and turbine noise. Moreover, the present invention provides an exhaust eductor system of minimal weight, compact enough for and configured for aircraft use, and which operates at low back pressure.
In one aspect of the present invention, an exhaust eductor system includes an oil cooler, an eductor primary nozzle downstream of a gas turbine engine, such as an APU in a commercial aircraft, an oil cooler air plenum downstream of the oil cooler for collecting oil cooling air and connected to an oil cooler air nozzle disposed about the eductor primary nozzle. A surge air plenum collects surge air from a surge air duct and is connected to a surge air dump nozzle disposed about the oil cooler air nozzle and directed into the eductor mixing duct.
The primary nozzle, oil cooler air nozzle, and surge air dump nozzle are formed so as to direct an exhaust flow from the gas turbine engine and entrain oil cooling air and surge air along with the exhaust flow in a direction having both radial and axial components with respect to a centerline axis of the gas turbine engine. The exhaust flow is directed into an eductor mixing duct angled away from the centerline axis of the gas turbine engine and then enters an exit duct angled toward the centerline axis of the gas turbine engine so that (1) direct line of sight acoustic paths from the tail pipe exit to the turbine exit are blocked, enhancing engine noise suppression, and (2) the exhaust flow is turned, further enhancing the performance of the acoustically treated duct surfaces. The tail pipe eductor mixing ducts and exit ducts are acoustically treated with bulk or reactive liners on some or all of the duct surfaces.
In another aspect of the present invention, a method may include steps of mounting an APU in an aircraft compartment and drawing cooling air through the compartment, in the process of providing compartment air cooling and gas turbine casing cooling, and further includes the steps of mounting a primary nozzle downstream of a gas turbine engine, collecting oil cooling air in an oil cooler air plenum, connecting an oil cooler air nozzle downstream of the oil cooler air plenum, disposing the oil cooler air nozzle about the primary nozzle, collecting surge air in a surge air plenum, connecting a surge air dump nozzle downstream of the surge air plenum, and mounting the surge air dump nozzle about the oil cooler air nozzle.
Then, the oil cooling air is entrained through the oil cooler air plenum and the oil cooler air nozzle, and the surge air is entrained through the surge air plenum, along with the exhaust flow from the gas turbine engine through the primary nozzle, in a direction having both a radial component and an axial component with respect to a centerline axis of the gas turbine engine. The oil cooling air is drawn across an oil cooler in the process providing cooling for the oil used in generators, and gearboxes. The exhaust flow, including the oil cooling air and surge air, is directed into an eductor mixing duct which is angled away from the centerline axis of the gas turbine engine, and is then turned into an exit duct which is angled toward the centerline axis of the gas turbine engine, in order to acoustically block core noise and reduce the acoustic energy of the exhaust flow.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.